By wind driven action on the ocean surface, sea spray aerosols (SSA) are generated and released to the atmosphere. SSA are enriched in organic matter and ions relative to bulk seawater. Particularly, calcium (Ca2+) has been shown to be enriched in fine and coarse SSA, and in the sea surface microlayer (SSML). This is likely due to an interaction with surface active fatty acids which are transferred as organic coatings on SSA during the bubble bursting process. Palmitic acid (PA) is among the most abundant fatty acids in the SSML and SSA, and is used here as a model system for sea surface films. In this study, the impact of calcium enrichment on the stability and protonation state of PA monolayers is investigated through the use of surface tensiometry and surface-sensitive spectroscopic techniques. The stability of a monolayer can be defined through its equilibrium spreading pressure (ESP). For [Ca2+] ≤ 1 mM, PA exhibits enhanced stability (ΔΠ = 7.1 mN/m for 1 mM Ca2+) relative to pure water. However, for [Ca2+] ≥ 10 mM, PA passes through a metastable state of greater stability prior to destabilizing and approaching surface pressures of 0 mN/m. Infrared reflection absorption spectroscopy (IRRAS) is used to determine the binding motif of Ca2+ to the carboxylate headgroup of PA over a range of concentrations (10 μM-600 mM). Additionally, IRRAS is used to track monolayer coverage during the course of the surface pressure decline of PA on a 300 mM CaCl2 subphase during the ESP measurement. Through this work, it has been determined that Ca2+ at marine relevant concentrations will bind to and deprotonate PA molecules at the interface, causing changes to the film stability, which can impact the climate relevant properties of aerosols, such as lifetime, reflectivity, and reactivity.

Impact of calcium enrichment on the stability of model sea surface films

254th National Meeting (2017)

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